Method of treating metal powders



?atented Jan. 19, 1954 METHOD OF TREATING METAL POWDERS Mark N.Fredenburgh, Summit, N. J., assignor to Radio Corporation of America, acorporation of Delaware No Drawing. Application January 31, 1950, SerialNo. 141,580

3 Claims.

The present invention relates to metal powders employed in making analloy by the powder metallurgy process including at least one refractorymetal, and to a method of treating said powders for improving theefficiency of said process.

Alloys including at least one refractory metal in large percentagesrequire recourse to the Well known powder metallurgy process for theirformation because of the relatively high melting point of refractorymetals. It is not feasible to make the alloy by fusing such metalstogether. According to the powder metallurgy process, powders of themetals to be included in a desired alloy are mixed together, placed in aform of predetermined dimensions and compressed under relatively highpressure to form slugs that are sufiiciently coherent for handling. Theslugs so formed are then sintered at a temperature sufficiently high toincrease their mechanical strength; for it is desirable that the slugsformed by the compression and sintering steps have good coherence andpredetermined dimensions for subsequent processing at high temperature.After the high temperature treatment causing the formation of the alloythe slugs of alloy are mechanically worked by swaging after which theyare drawn through several successive dies of diminishing aperture sizeto successively reduce the resultant rods into progressively finer wireformations.

While commercially available metal powders have the desired chemicalpurity for good results in making the alloy wire, the physicalcharacteristics of such powders, particularly tungsten powder, varyconsiderably from lot to lot. Since the conditions, such as pressure,temperature and time are usually standardized and fixed in a powdermetallurgy process, for the preparation of a specific product variationsin the physical characteristics of the powders forming the productresult in a sacrifice of efiiciency in carrying out the process. Onemethod of measuring the eii'iciency of a powder metallurgy processinvolving the manufacture of wire is to compare the ratio of the mass ofthe finished alloy wire to the mass of the powders employed in makingthe alloy wire. In many cases, small variations in the physicalcharacteristics of the powders may alter the results to such an extentthat the manufacturing efiiciency is reduced to a figure of -50 percent.

Weak slugs that fracture during swaging of the alloy may resultgenerally from certain physical characteristics of the metal powdersemployed in making the auoy by a fixed process.

One respect in which the physical characteristics of the metal powdersvary is in their apparent density. This is distinguished from theabsolute density in that it includes in the volume of the powders thepores and spaces between the particles of the powders. As a consequenceof this variation in apparent density, when a form or receptacle is usedin the practice of the powder metallurgy process having predetermineddimensions for receiving the powders and in which the powders arecompressed, the slugs resulting from the application of a definitepressure may be of different sizes and each may have a differentapparent density. Also, such variations in apparent density materiallyaffect the efficient loading of the powder receptacle, which issubsequently reflected in the overall efliciency of the process.

Another characteristic of commercial metal powders that varies from lotto lot of such powders is the particle size range and the particle sizedistribution in the range. Commercial powders such as the powders oftungsten and molybdenum as used in the preparation of alloy wire areusually composed of particles the size of which varies from slightlyabove 0 to 10 microns in diameter. While this range is satisfactory fora reasonably efficient powder metallurgy process, the particle sizedistribution in per cent by number sometimes peaks near the lowest limitof the range and sometimes near the highest limit of the range, each ofwhich conditions adversely affects the efiiciency of the processreferred to.

For example, if the particles peak in size near the lowest limit of thesize range in the powder of one metal and peak at a different portion ofthe range in the powder of another metal to be used in making an alloy,difficulty is experienced in suitably mixing the powders. In order to secure a homogeneous mixture, it is necessary that the particles to bemixed be relatively closely related in size.

Another objection to a peaking of the particle size distributionadjacent the lowest limit of the range is that a slug formed therefromduring the first steps of the powder metallurgy process, is relativelyweak and breaks easily during subsequent handling resulting in a wasteof metal powder.

On the other hand if the particle size distribution of one metal powderto be mixed with another metal powder, peaks near the highest limit ofthe range a problem of mixing is also created where the other powderpeaks away from the highest limit of the range. Another objection topowders peaking at the highest limit of the range is that increaseddifficulties are encountered during the subsequent stages of the wireprocess involving swaging and drawing which also result in lowering theefiiciency of the process. While it is feasible in the prior art todetermine by tests or actual measurements the limits of the particlesize range of a powder and the apparent density thereof, there are nosimple and satisfactory means available for determining with sufficientaccuracy the distribution of particle size fractions so that theperformance of thepowder can be accurately pred i,cte d. The only waythis could be determinedwasby. actual per.-

formance tests according to which a sample of a powder was subjected tothe usual steps of the powder metallurgy process and the value of; thepowder determined by the efliciency of the process.

This is entirely satisfactory for material:

bodies of increased resistance to fracture during test and. a treatmentof said powder based on said test for providing a powder having aparticle size peaking at a predetermined portion of the particle sizerange.

producing good efficiency but involves considerable cost and requiredrepetition for establishing the value of the material from lot to lotand no disposition of unusable material such as altering or adjustingits physical characteristics to produce satisfactory performance, wasavailable.

This lack in sumcient uniformity in physical characteristics ofcommercial metal powders, is a serious disadvantage in presentmanufacturing pr cedures for making certain alloys by the powdermetallurgy process. Such procedures are usually standardized to providea fixed value of pressures, temperatures and duration of applications.Thus for one value of these manufacturing conditions I have found inmaking alloys of tungsten and molybdenum that a particle size of thepowders ranging from slightly above to microns in diameter and peakingat from 1 to 2 microns is particularly suitable for carrying out anefiicient alloying process. It, is extremely diflicult to obtaincommercial powders conforming to a narrow range specification. However,a powder having at leastv 95 per cent of the particles fromslightlyabove O to 3 microns in diameter and with the remainder of theparticles not over 10 microns can, be readily supplied by the averagemanufacturer. This powder may or may not be satisfactory in, itscommercial form for an efficient powder metallurgy. process since theparticles may actually have a size distribution that peaks at say fromslightly above 0 to 1 micron, from 1-2 microns or from 2 to 3 microns.While powders of these characteristics would satisfy the foregoingspecification they would not necessarily lend themselvesto an efficientalloying and wire making process controlled by the standardized valuesreferred to.

Accordingly it is the object of theinvention to improve the efficiencyof av powder metallurgy process.

Another object is to providemetal powders of uniform physicalcharacteristics for improving the efficiency of a process for making analloy therefrom.

A further object is to provide a novel method of treating commercialmetal powders composed of definite particle size fractions so as to.alter and adjust the particles to producepowders having new definiteparticle size fractions suitable for use in an efficient alloyingprocess.

Another object is to provide a method oftreating commercial metalpowders to reduce their particle size range for accommodating thepowders to standardized manufacturing procedures with increasedefiiciency.

A further object is to provide a metal powder that contributes to theformation of coherent According to one aspect of the inventioncommeroial.powders of metal such as tungsten and molybdenum are firsttested microscopically to determine the limits of the particle sizerange thereof. If the test indicates that this range is between 0 and 10microns, the powder is of a type that can be suitably treated by themethod of the invention. No preliminary test is neces cary of theapparent density of the powder since the treatment according to theinvention renders substantially uniform the apparent density of powdershaving originally different apparent densitiesl Essentially thetreatment of commercial metal powders according to the invention forimprov ing the efiiciency of a powder metallurgy process for making analloy of the metals of the powder, includes further limiting the sizerange of the particles. This further limitation is accomplished byenlargement of the smaller particles and reduction in size of the largerparticles. The treatment involves first heating the powders in areducing atmosphere such as dry hydrogen at a predetermined temperatureand for a predetermined time duration. The temperature and time ofapplication are controlled to permit a sintering of the smallerparticles to form-relatively larger particles. Substantially no change 4occurs in the larger particles as a result of the heat treatment. Theheat treated powder is then subjected to mechanical. working to reducethe size of the sintered particles and the size of the agglomerates,originally present in the powder. According to the invention a predetemined value for the factors involved in the heat treatment and in themechanical working will provide a resultant powder having apredetermined. particle size range and a predetermined peak of the sizedistribution in said range.

For best results in a powder metallurgy process involving a specificproduct it is generally believed the particle size should belimited to adefinite range, with the size distribution peaking at some intermediatepoint in the range. For example, one group of investigators believe apeak at a size of from 4 to 5 microns in diameter is most suitable formaking an alloy. Others believe the peaking point should be at othertions of the range. I have found for example that a metal powder havinga particle size of from slightly above 0, to 10 microns in diameter andpeaking at a. particle size of from 1 to 2 microns is well suited formaking an alloy with aminimum of powder loss. However, my: method oftreating commercial metal powders may be practiced with equal advantageno matter what peaking point has been found desirable. For example, asatisfactory powder for improving the efficiency. of a powder metallurgyprocess may include particles in an overall size range of from slightlyabout zero to 8 or microns in diameter with the size distributionpeaking in a sub range of from 0.5 to 5 microns.

I have practiced my invention in the manufacture of a tungstenmolybdenum alloy known as Dowmo and wherein the commerical powders ofthe metals were supplied in response to a specification calling for aparticle size range of assuming it is desired to mix the powders priorto the heat treatment. As an alternative the powders may be mixed afterthe heat treatment, in which event, the initial mechanical working isnot required.

The resultant powders are found to be of reduced porosity, of increasedapparent density and to have a particle size range of from slightlyabove zero to slightly above 8 microns in difrom 0 to 10 microns indiameter with 95 per 10 ameter with the particle size distributionpeakcent of the particles being from 0 to 3 microns ing at from 1 to 2microns in diameter. in diameter. The 0 valueindicates a magnitude Ihave found that commercial molybdenum that is so small as to beunmeasurable and relapowders are more predictable in their physicaltively close to 0 microns. characteristics than commercial tungsten pow-In carrying out my process for treating comders and consequentlysatisfactory alloying methmercial powders, the powders supplied in acadshave been practiced in reliance on suppliers cordance with the foregoingspecification are specification for molybdenum powders. In makfirsttested by the use of an electron microscope ing a Dowmo alloy thereforethe treatment acto determine whether the limits of the size cording tothe invention may be applied solely range are as specified. to thetungsten powder. Since commercial The next step in the method accordingto the tungsten powders require my treatment to a invention is to heatthe metal powders referred greater extent than other powders, I havepracto in the preceding paragraph. As actually carticed the invention byusing tungsten powders ried out the heat treatment may involve the fromseveral different sources having a relatively use of molybdenum boats 10long, 1 /8 wide wide difierence in their physical characteristics. and1" deep, into which the metal powders are The following table indicatesthe particle size loaded. The boats are tapped during the loading rangeand the peaking point of the particle size to pack the powders and thepowders are levelled distribution inconnection with several of such ofiat the tops of the boats. Each boat concommercial tungsten powders aftertreatment by tains either tungsten powder, molybdenum my method.

TABLE Tungsten powders processed according to the invention ParticleSize Range in Microns, Percent Commercial Designation of Tungsten PowderOallite 466E 15.6 34.4 37.5 10.3 1.7 N. A. Philips 0-144-1 14.9 23.745.3 14.9 1.2 06.111 6 466D101-l 9.4 26.3 46.3 14.5 3.0 0611116 432 12.017.3 54.3 13.5 2.4 N. A. Philips 0-144-2.. 15.8 12.2 44.9 20.8 5.1RandR48l-1-Hl2-.... 27.2 3.3 22.6 23.9 11.3 Rand R48l-l-H2 3.4 4.2 28.534.6 19.1 Rand R48l-Hl 7.6 3.3 19.3 36.8 23.1

powder, or a mixture of tungsten and molybdenum powders, and the boatsare heated individually. Boats containing tungsten powder may be heatedfor one half hour at a temperature of 1700 degrees C. B. T.i25 degreesC. using an optical pyrometer for temperature measurement. Boatscontaining molybdenum powder are heated at a lower temperature, to wit1500 degrees C. B. T. Boats containing mixtures of tungsten andmolybdenum powders in the heretofore known relative amounts forproviding the alloy known as Dowmo, are heated for one half hour at atemperature of 1600 degrees C. B. T.i25 degrees C. The heat treatmentsreferred to are conducted in dry hydrogen. The resultant product is inthe form of slugs of sintered metal.

The next step in the novel method of the in-, vention is to crush thesintered slugs as by means of a jaw crusher so that all of the materialis reduced to particle sizes capable of passing through a No. 8 standardtesting sieve. The mixture so crushed is then mechanically worked as byball milling in porcelain jars of one gallon capacity and containing3000 grams of flint pebbles of from A" to 1 diameter, rotated at a speedof R. P. M. for about 3 hours.

Before the first step is carried out, the commercial powders may bemechanically Worked for about three hours to thoroughly mix the powders,

While the tungsten powders commercially designated Callite 466-131, N.A. Philips 014=4-1, Callite 466-D1014, Callite 482, and N. A. Philips0444-2 have a distribution range after treatment according to theinvention that peaks at between a particle size of from 1 to 2 microns,which I have found particularly advantageous, the others represent animprovement over the commercial powders. If further improvement isdesired in the other powders the heat treatment step temperature may beraised in the direction indicated by the particle size distribution.

I have found that the temperature of the heat treatment according to theinvention for tungsten may be carried out with best results from about1600 degrees C. to about 1750 degrees C. without change in the time ofapplication, although the individual temperatures in this range wouldchange the location of the particle size distribwtion peak to differentpoints on the size range. An increase in temperature increases thesintering action so that the bond between the particles becomes strongerand the agglomcrates are more diificult to break up in the mechanicalworking step. The temperature therefore should not be high enough tomake this bond excessively strong since such particles would be verydifficult to break up. Another variable in the heat treatment accordingto the invention is the time 7 duration of its application. The timeduration may be either increased or reduced from the thirty minuteperiod specified, for either extending or contracting the lower limit.of theparticle size range of the powder.

While, as has been indicated, it is feasible according to the inventionto mix. commercial powders of metals which it is desired to alloy, suchfor example, as the powders. of tungsten and molybdenum, prior to theheating step, it is preferable to subject the powders to my novel heattreatment prior to the mixing step. This for the reason that a morehomogeneous mixture is secured when the range of particle size iscontracted to reduce the maximum difference in particle size.

The invention may be practiced by employing a relatively wide range inthe temperature, and duration of its application, of the heattreatmentaccording to the invention, as well as in the time duration of thesubsequent mechanical working operation. For example, a temperature solow and applied for such a short time as to cause only a slightsintering of the powder particles involves a practice of the treatmentof the invention. is is for the reason that such slight sintering actionaccomplishes at. least a partial agglomeration of the smaller powderparticles and this serves to contract the range of the par ticle size ofthe powder. Moreover, amechanical working may be accomplished accordingto the invention by subjecting the powder after the heat treatment, orwithout the heat treatment. to a ball milling operation for a fractionof the three hours specified in the example described in detail beforeherein. A longer time than three hours may also be used in accordancewith the invention.

It is therefore to be understood that the invention in its broaderaspects it not to be regarded limited to the values set Iorth in the.

example referred to, but to be directed to a methed for reducing theparticle size range of metal powders and to cause a predominant portionof the particles to lie in a relatively narrow size sub-range inrelation to the overall size range. While, as has been indicated above,I have found a particle size range of from slightly above to 8 micronswith the size distribution peaking at a sub-range of from 1 to 2 micronsmost advantageous in promoting efilciency of a powder metallurgyprocess, the peaking of the size distribution in anothersnb-rangewithinthe overall size range may also be accompanied by some 7increase in efficiency in making the alloy over that secured by usinguntreated commercial POWLBlS.

The treatment according to the invention not only results in a desirablesize range and size distribution but also in an increase in the apparentdensity of the particles. This is important as has been previouslymentioned in that increased apparent density increases the strengthvwith which the slugs lormed of the powder are held together and resistbreakage during handling or processingby the powder metallurgy process.According to the invention, the apparent density is first increasedduring the heating step when a substantial number of the pores in theoriginal particles are closed by the sintering togeth of the particles.The apparent density is further increased by the mechanical working stepduring which the particles are rubbed together with an appreciableforce. Not only is the apparent density increased in accordance with theinvention but the apparent. density of several lots of powders treated.by me were. found to have a uniform apparent density although theapparent density of the original commercial powders was non-uniform.This uniformity in apparent density is important as has been pointed outbefore herein.

t will be apparent from the foregoing that I have provided anadvantageous treatment as a substitute for a lengthy and uneconomicalperformance test for assuring suitability of a commercial metal powderfor use in an efficient powder metallurgy process.

While I have indicated several ways in which the invention can becarried out it is to be understood that the invention is capable of manymodifications as will appear to persons skilled in the art withoutdeparting from the spirit thereof and it is desired to include thesemodifications within the scope of the appended claims.

I claim:

1. In a method of making a relatively strongly coherenttungsten-molybdenum alloy slug from commercial tungsten powder havingunpredictable characteristics in respect of particle size distributionand size distribution peak and having relatively small and relativelylarge particles, and commercial molybdenum powder having. a predictableparticle size distribution of from slightly above zero to 10 microns indiameter and a size distribution peak of from one-half to five micronsin diameter, said method comprising the steps of heating said commercialtungsten powder at a temperature from about 1660 to about 1750 C. forone-half hour to cause said relatively small particles of said tungstenpowder to become sintered together in a relatively strongly coherentbond and to cause said relatively large tungsten particles to becomesintered in a relatively weakly coherent bond, and subsequentlymechanically working said sintered tungsten powder for about three hoursto release said larger particles only from said relatively weaklycoherent bond, for providing a treated tungsten powder having saidpredictable particle size distribution and size distribution peak,whereby said treated tungsten powder and said commercial molybdenumpowder are adapted to be homogeneously mixed and sintered to providesaid strongly coherent slug.

2. Method of making a drawn wire of an alloy of tungsten and molybdenum,from commercial powders of said metals, comprising the steps ofseparately heating predetermined amounts of said tungsten and molybdenumpowders to sinter the particles thereof, separately mechanically workingthe sintered particles to provide treated powders having smaller sizeranges than said commercial powders, mixing said treated powders to forma homogeneous mixture, pressing said mixture to slug form, heating saidslug form to sinter the particles thereof to provide a strongly anduniformly coherent body, and drawing said body to relatively fine wireform.

3. Method of making a drawn wire of an alloy of tungsten and molybdenumfrom commercial powders of said metals, comprising heating apredetermined amount of said tungsten powder at a temperature from about1660 to 1750" C. for one-half hour, to cause the particles thereof tosinter together in relatively weak bonds, mechanically working thesintered tungsten powder for about three hours, whereby a treatedtungsten powder is provided havinga reduced amount of form to provide astrongly coherent body, and

drawing said body to wire form.

MARK N. FREDENBURGH.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,697,402 Nutter et a1. Jan. 1, 1929 1,915,386 Schumacher eta1. June 27, 1933 Number Name Date 2,082,126 Schulz June 1, 19372,199,191 Tour Apr. 30, 1940 2,294,895 Drapeau et al. Sept. 8, 19422,306,665 Schwarzkopf Dec. 29, 1942 2,359,401 Wulff Oct. 3, 19442,448,243 Anderson Aug. 31, 1948 2,461,089 'Smidth Feb. 8, 19492,464,517 Kurtz Mar. 15, 1949 OTHER REFERENCES Treatise on PowderMetallurgy, vol. I, page 239. Edited by Coetzel. Published byInterscience Publishers, 1110., New York.

1. IN A METHOD OF MAKING A RELATIVELY STRONGLY COHERENT TUNGSTEN-MOLYBDENUM ALLOY SLUG FROM COMMERCIAL TUNGSTEN POWDER HAVING UNPREDICTABLE CHARACTERISTICS IN RESPECT OF PARTICLE SIZE DISTRIBUTION AND SIZE DISTRIBUTION PEAK AND HAVING RELATIVELY SMALL AND RELATIVELY LARGE PARTICLES, AND COMMERCIAL MOLYBDENUM POWDER HAVING A PREDICTABLE PARTICLE SIZE DISTRIBUTION OF FROM SLIGHTLY ABOVE ZERO TO 10 MICRONS IN DIAMETER AND A SIZE DISTRIBUTION PEAK OF FROM ONE-HALF TO FIVE MICRONS IN DIAMETER, SAID METHOD COMPRISING THE STEPS OF HEATING SAID COMMERCIAL TUNGSTEN POWDER AT A TEMPERATURE FROM ABOUT 1660* TO ABOUT 1750* C. FOR ONE-HALF HOUR TO CAUSE SAID RELATIVELY SMALL PARTICLES OF SAID TUNGSTEN POWDER TO BECOME SINTERED TOGETHER IN A RELATIVELY STRONGLY COHERENT BOND AND TO CAUSE SAID RELATIVELY LARGE TUNGSTEN PARTICLES TO BECONE SINTERED IN A RELATIVELY WEAKLY COHERENT BOND, AND SUBSEQUENTLY MECHANICALLY WORKING SAID SINTERED TUNGSTEN POWDER FOR ABOUT THREE HOURS TO RELEASE SAID LARGER PARTICLES ONLY FROM SAID RELATIVELY WEAKLY COHERENT BOND, FOR PROVIDING A TREATED TUNGSTEN POWDER HAVING SAID DISTRIBUTION PEAK, WHEREBY SAID TRIBUTION AND SIZE DISTRIBUTION PEAK, WHEREBY SAID TREATED TUNGSTEN POWDER AND SAID COMMERCIAL MOLYBDENUM POWDER ARE ADAPTED TO BE HOMOGENEOUSLY MIXED AND SINTERED TO PROVIDE SAID STRONGLY COHERENT SLUG. 